1. Field of the Invention
The present disclosure relates to a large area organic light emitting diode display and a method for manufacturing the same. Especially, the present disclosure relates to a large area organic light emitting diode display having color light emitting diode in each pixel and a method for manufacturing the same with a photo lithography process.
2. Discussion of the Related Art
Nowadays, various flat panel display devices are developed for overcoming many drawbacks of the cathode ray tube such as heavy weight and bulk volume. The flat panel display devices include the liquid crystal display device (or LCD), the field emission display (or FED), the plasma display panel (or PDP) and the electroluminescence device (or EL).
The electroluminescence display device is categorized in the inorganic light emitting diode display device and the organic light emitting diode display device according to the luminescence material. As a self-emitting display device, the electroluminescence display device has the merits that the response speed is very fast, the brightness is very high and the viewing angle is large.
FIG. 1 is a diagram illustrating the structure of the organic light emitting diode. As shown in FIG. 1, the organic light emitting diode comprises the organic light emitting material layer, and the cathode and the anode which are facing each other with the organic light emitting material layer therebetween. The organic light emitting material layer comprises the hole injection layer HIL, the hole transport layer HTL, the emission layer EML, the electron transport layer ETL and the electron injection layer EIL. The organic light emitting diode emits light due to the energy from the exciton formed at the excitation state in which the hole and the electron are recombined at the emission layer EML.
The organic light emitting diode emits light due to the energy from the exciton formed at the excitation state in which the hole from the anode and the electron from the cathode are recombined at the emission layer EML. The organic light emitting diode display can represent the video data by controlling the amount (or ‘brightness’) of the light generated and radiated from the emission layer ELM of the organic light emitting diode as shown in FIG. 1.
The organic light emitting diode display (or OLED) using the organic light emitting diode can be categorized in the passive matrix type organic light emitting diode display (or PMOLED) and the active matrix type organic light emitting diode display (or AMOLED).
The active matrix type organic light emitting diode display (or AMOLED) shows the video data by controlling the current applying to the organic light emitting diode using the thin film transistor (or TFT).
FIG. 2 is the exemplary circuit diagram illustrating the structure of one pixel in the active matrix organic light emitting diode display (or AMOLED). FIG. 3 is a plane view illustrating the structure of one pixel in the AMOLED. FIG. 4 is a cross sectional view along the cutting line I-I′ for illustrating the structure of the AMOLED.
Referring to FIGS. 2, 3 and 4, the active matrix organic light emitting diode display comprises a switching thin film transistor ST, a driving thin film transistor DT connected to the switching thin film transistor ST, and an organic light emitting diode OLED connected to the driving thin film transistor DT. By a scan line SL, a data line DL and a driving current line VDD disposed on a substrate SUB, a pixel area is defined. The organic light emitting diode OLED is formed in one pixel area and defines a light emitting area within the pixel area.
The switching thin film transistor ST is formed where the scan line SL and the data line DL cross. The switching thin film transistor ST acts for selecting the pixel which is connected to the switching thin film transistor ST. The switching thin film transistor ST includes a gate electrode SG branching from the scan line SL, a semiconductor channel layer SA overlapping with the gate electrode SG, a source electrode SS and a drain electrode SD. The driving thin film transistor DT acts for driving an anode electrode ANO of the organic light emitting diode OLED disposed at the pixel selected by the switching thin film transistor ST. The driving thin film transistor DT includes a gate electrode DG connected to the drain electrode SD of the switching thin film transistor ST, a semiconductor channel layer DA, a source electrode DS connected to the driving current line VDD, and a drain electrode DD. The drain electrode DD of the driving thin film transistor DT is connected to the anode electrode ANO of the organic light emitting diode OLED. Between the anode electrode ANO and the cathode electrode CAT, the organic light emitting layer OLE is disposed. The cathode electrode CAT is connected to the base voltage VSS. Between the gate electrode DG of the driving thin film transistor DT and the driving current line VDD or between the gate electrode DG of the driving thin film transistor DT and the drain electrode DD of the driving thin film transistor DT, a storage capacitance Cst is formed.
Referring to FIG. 4 more detail, on the substrate SUB of the active matrix organic light emitting diode display, the gate electrodes SG and DG of the switching thin film transistor ST and the driving thin film transistor DT, respectively are formed. On the gate electrodes SG and DG, the gate insulator GI is deposited. On the gate insulator GI overlapping with the gate electrodes SG and DG, the semiconductor layers SA and DA are formed, respectively. On the semiconductor layer SA and DA, the source electrode SS and DS and the drain electrode SD and DD facing and separating from each other are formed. The drain electrode SD of the switching thin film transistor ST is connected to the gate electrode DG of the driving thin film transistor DT via the contact hole penetrating the gate insulator GI. The passivation layer PAS is deposited on the substrate SUB having the switching thin film transistor ST and the driving thin film transistor DT.
The upper surface of the substrate having these thin film transistors ST and DT is not in even and/or smooth conditions, but in uneven and/or rugged conditions having many steps. In order for the organic light emitting diode display to have good luminescent quality over the whole display area, the organic light emitting layer OLE should be formed on an even or smooth surface. So, to make the upper surface in planar and even conditions, the over coat layer OC is deposited on the whole surface of the substrate OC.
Then, on the over coat layer OC, the anode electrode ANO of the organic light emitting diode OLED is formed. Here, the anode electrode ANO is connected to the drain electrode DD of the driving thin film transistor DT through the contact hole penetrating the over coat layer OC and the passivation layer PAS.
On the substrate SUB having the anode electrode ANO, a bank BANK is formed over the area having the switching thin film transistor ST, the driving thin film transistor DT and the various lines DL, SL and VDD, for defining the light emitting area. The exposed portion of the anode electrode ANO by the bank BANK would be the light emitting area. On the anode electrode ANO exposed from the bank BANK, the organic light emitting layer OLE is formed. On the organic light emitting layer OLE, the cathode electrode ACT is formed.
For the case of the bottom emission type full color organic light emitting diode display, a color filter CF may be further comprised between the over coat layer OC and the passivation layer PAS, and the anode electrode ANO may include the transparent conductive material. For this case, the organic light emitting layer OLE may have the white light emitting organic material. And, the organic light emitting layer OLE and the cathode electrode CAT may be deposited as covering the whole surface of the substrate SUB.
On the other hand, for the case of the top emission type full color organic light emitting diode display, the anode electrode ANO may be made of a reflective electrode. The organic light emitting layer OLE may have the organic material radiating any one color light of red, green and blue color. The cathode electrode CAT may be deposited as covering the whole surface of the substrate SUB. For other example, the organic light emitting layer OLE may have the white light emitting organic material. In this case, the organic light emitting layer OLE and the cathode electrode CAT may be deposited as covering the whole surface of the substrate SUB. And, on the organic light emitting layer OLE or the cathode electrode CAT, the color filter CF may be formed.
As the organic light emitting diode display is one of the self-luminescence device, the back light unit used for the liquid crystal display is not required. Therefore, the organic light emitting diode display is more preferable to develop a thin flat type display. Furthermore, as it has the low light energy loss, it is easy to develop brighter display with lower power consumption. With many merits, the organic light emitting diode display would be the best solution for the next generation display. However, the mass manufacturing method for large area display having the 20 inch or more diagonal area such as TV monitor has not been developed. Until now, the organic light emitting diode display is applied to the small display having at most 15 inch or less diagonal display area.
In order to manufacture a full color organic light emitting diode display, it is important to form an organic light emitting layer having any one color of red, green and blue colors at each pixel area, selectively. However, it is very hard to pattern the organic light emitting material by the currently used photo lithography method, while ensuring a stable and reliable pattern quality. One possible method has been suggested in which the organic light emitting material is selectively deposited on the large area substrate using a fine patterned screen mask.
When the display size of the organic light emitting diode display is less than 15 inch diagonal area, the organic light emitting layer OLE can be selectively formed on the anode electrode ANO. For example, with a simple process using a screen mask, the organic light emitting layer OLE can be formed within a pre-determined area. However, when the display size is larger than 20 inches, it is almost impossible to form the organic light emitting layer within specified areas over a large area using the screen mask process. This is because as the size of the display area increases, the size and weight of the screen mask also increases. The additional size and weight of the screen mask causes the screen mask to sag in the middle so that it becomes non-planar. As a result, it is hard to deposit the organic light emitting material with uniform thickness over the large area substrate with a screen mask. Consequently, a new method for manufacturing a large area organic light emitting diode display with organic light emitting materials radiating red, green and blue light colors respectively is required.
Furthermore, in order to accomplish a mass production method for manufacturing the large area organic light emitting diode display, there are many problems to be overcome.